The present invention relates to a variable compression ratio mechanism for a reciprocating internal combustion engine, and particularly to the improvements of a lower link of a multi-link type reciprocating internal combustion engine, rotatably installed on a crankpin.
In recent years, there have been proposed and developed various variable compression ratio mechanisms for reciprocating internal combustion engines, in which a compression ratio is variable depending upon engine operating conditions, such as engine speed and load. One such variable compression ratio mechanism has been disclosed in Japanese Patent Provisional Publication No. 2000-73804 (hereinafter is referred to as xe2x80x9cJP2000-73804xe2x80x9d). JP2000-73804 discloses a multi-link type variable compression ratio mechanism that a piston and a crankshaft are mechanically linked to each other via a plurality of links. Briefly explaining, the multi-linked variable compression ratio mechanism of JP2000-73804 includes an upper link, a lower link, and a control link. One end of the upper link is rotatably connected to a piston via a piston pin. The other end of the upper link is rotatably pin-connected to the lower link by means of a first connecting pin. The lower link is rotatably installed onto a crankpin of an engine crankshaft. One end of the control link is rotatably connected to the lower link by means of a second connecting pin. The other end of the control link is rotatably connected onto an eccentric cam of a control shaft. The position of the axis of the eccentric cam relative to the axis of the control shaft, that is, the center (pivot axis) of oscillating motion of the control link shifts or displaces relative to the engine body (a cylinder block) by rotating the control shaft by means of an actuator such as an electric motor. As a result of this, a condition of restriction of motion of the lower link via the control link changes, and thus a crank angle versus piston stroke characteristic (containing the position of TDC), that is, a compression ratio varies. Generally, the lower link has a two-split structure composed of a main lower-link portion and a lower-link bearing cap portion separable from each other, so that the lower link can be installed onto or removed from the crankpin. The main lower-link portion and the lower-link bearing cap portion are integrally connected by means of bolts. The substantially half-round section of the main lower-link portion and the substantially half-round section of the lower-link bearing cap provide or form a cylindrical crankpin bearing, when these two halves are assembled to each other with bolts. The main lower-link portion is also formed with a first connecting-pin bearing portion into which the first connecting pin is inserted and a second connecting-pin bearing portion into which the second connecting pin is inserted. As viewed in a direction perpendicular to the axial direction of the crankpin, each of the first and second connecting-pin bearing portions is formed as a forked end, so that each connecting pin is supported at its both axial ends by means of the forked end composed of a pair of axially-spaced connecting-pin supports or a pair of axially-spaced connecting-pin bearings.
In the multi-linked variable compression ratio mechanism of JP2000-73804, input load is transferred from the upper link and/or the control link and then acts on the lower link via the first connecting pin and/or the second connecting pin. At this time, the input load is further transferred from the two axially-spaced connecting-pin bearings of the forked end of each connecting-pin bearing portion, and acts directly on axial ends of the cylindrical crankpin bearing (see FIG. 9A). There is a possibility that two axial ends of the cylindrical crankpin bearing are remarkably deformed due to the input load. The crankpin bearing is a slide bearing that supports the load by virtue of the films of lubricating oil. In such slide bearings, there is a tendency that the pressure of the lubricating oil film in the crankpin bearing is relatively high at the axial central portion of the crankpin bearing. On the other hand, the pressure of the lubricating oil film in the crankpin bearing is released at the axial end of the crankpin bearing and thus the pressure of the lubricating oil film is relatively low at the axial end. For the reasons discussed above, if the two axial ends of the cylindrical crankpin bearing deform owing to the input load, the input load may not be satisfactorily supported by virtue of the pressure of the oil film. Therefore, there is a possibility of metal-to-metal contact between the axial ends of the crankpin bearing and the outer peripheral wall surface of the crankpin (the bearing journal portion). This results in extremely rapid wear and increased friction.
Accordingly, it is an object of the invention to provide a variable compression ratio mechanism for a reciprocating internal combustion engine, which avoids the aforementioned disadvantages.
It is another object of the invention to provide a variable compression ratio mechanism for a multi-link type reciprocating internal combustion engine employing an upper link, a lower link, and a control link, which is capable of effectively reducing deformation of axial ends of a crankpin bearing, which may occur owing to input load transferred from a lower-link connecting pin to a connecting-pin bearing portion of the lower link, suppressing the input load from concentratedly acting on the axial ends of the crankpin bearing.
In order to accomplish the aforementioned and other objects of the present invention, a variable compression ratio mechanism for a reciprocating internal combustion engine employing a reciprocating piston movable through a stroke in the engine and having a piston pin, and a crankshaft changing reciprocating motion of the piston into rotating motion and having a crankpin, comprises an upper link connected at its one end to the piston pin, a lower link connected to the other end of the upper link via a first connecting pin and rotatably installed on the crankpin, a control link connected at its one end to the lower link via a second connecting pin, and pivotably connected at the other end to a body of the engine to permit oscillating motion of the control link on the body of the engine, a control mechanism shifting a center of oscillating motion of the control link to vary a compression ratio of the engine, and the lower link comprising a crankpin bearing portion into which the crankpinis fitted, a first connecting-pin bearing portion, which is parallel to the crankpin bearing portion and into which the first connecting pin is fitted, a second connecting-pin bearing portion, which is parallel to the crankpin bearing portion and into which the second connecting pin is fitted, a central connecting portion having an axial length shorter than each of an axial length of the crankpin bearing portion, an axial length of the first connecting-pin bearing portion, and an axial length of the second connecting-pin bearing portion, and the central connecting portion that connects an axial central portion of at least one of the first and second connecting-pin bearing portions to an axial central portion of the crankpin bearing portion.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.